<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <style>
        .right {
            color: green;
            font-weight: 700;
        }
        .wrong {
            color: red;
            font-weight: 700;

        }
    </style>
</head>
<body>
    <input type="text" class="uname"> <span>请输入用户名</span>
    <script>
      var reg = /^[a-zA-Z0-9_-]{6,16}$/;
        var uname = document.querySelector('.uname');
        var span = document.querySelector('span');
        uname.onblur = function() {
            if(reg.test(this.value)) {
                span.className = 'right';
                span.innerHTML = '格式输入正确';
                
            } else {
                span.className  = 'wrong';
                span.innerHTML = '格式输入错误';
            }
        }
    </script>
</body>
</html>